1) Field of the Invention
This invention relates generally to the fabrication of semiconductor devices and more particularly to a method of fabricating twin tubs or wells in a silicon substrate.
2) Description of the Prior Art
In the fabrication of CMOS devices it is frequently desirable to make a complementary or symmetric environment with respect to the NMOS and PMOS devices. In other words, it is frequently necessary to create a suitable N-type region for the PMOS devices and a suitable P-type region for the adjacent NMOS devices. Each of these N-type and P-type regions is generally referred to as a "tub" or "well."
It is known that the formation of such N-wells and P-wells may be achieved by the implantation of an appropriate dopant species, such as boron or phosphorus, into a suitable substrate followed by the high temperature drive-in of the implanted ion. To manufacture advanced twin tub CMOS devices on almost intrinsic substrates with the known methods, starting from lightly doped at the limit intrinsic substrates, two tubs are formed with opposite conductivity type (P and N) in which N and P-channel transistors are respectively formed. In order to reduce latch-up problems (that is, switching on of parasite SCR structures) and to obtain more compact circuits, the two doping tubs are separated by a field oxide.
A typical process sequence uses a LOCOS-based approach to isolate like devices. The process is a two-mask, self-aligned LOCOS twin-well process with two separate well implants. First, a first photo resist layer is formed covering the p-well areas. Then n-type impurities are implant into the n-well areas and the first photo resist layer is removed. Next, a thick masking oxide (LOCOS) is selectively grown over the N-well areas. This masking oxide consumes a significant depth of the silicon surface and causes topography that can interfere with subsequent overlaying layers. The masking oxide (LOCOS) typically has a thickness in the range of between about 2000 and 6000 .ANG. and consumes a depth of the silicon substrate in the range of between about 1000 and 3000 .ANG.. Then, using the masking oxide (LOCOS) as an implant mask, p-impurities are implanted into the substrate to form p-wells. The masking oxide (LOCOS) is removed thereby forming depressions in the substrate surface. A nitride masking layer is deposited and patterned to cover the active areas using a second resist layer. Finally, the field oxide is formed over non-active area and overlaps the n and p-well borders. This process creates rugged topology by forming the LOCOS masking layer and the field oxide regions. The substrate surface is lower in the n-well region where the oxide masking layer (LOCOS) consumed the silicon substrate.
The known methods have several variations. However all these various embodiments have in common the use of distinct masking step for forming the masks for the N and P tubs. Moreover, many of these methods produce rough surface topologies that interfere with the layers and structures that overlie them. The topography differences require large depth of focus in lithography exposure which is very difficult to achieve and costly to manufacture. Other methods use thick photoresist to perform self-aligned twin-tub formation after the isolation process. However, the photoresist thickness control and across wafer uniformity, and shrinkage during implantation may endanger the depth as well as the uniformity of intended dopant profiles.
Workers in the art are aware of the problems of complicated process steps and excess surface topography and have attempted to resolve these problems. For example, U.S. Pat. No. 4,525,920 (Jacobs et al.) teaches a method of twin tub formation where a photo resist layer defines the N-tub region. A thermal oxide layer is used to mask the P-tub region. U.S. Pat. No. 4,707,455 (Tsang et al.) teaches a method of twin tub formation where two photoresist layers are used to mask the N and P-tub regions. Then a field oxide (FOX) region is formed and Boron is implanted through the FOX on the P-tub side. However, this method is not self aligning and requires two masks. U.S. Pat. No. 4,435,895 (Parrillo et al.) teaches a self-aligned tub process where the N tub is defined by a nitride/oxide mask. Then a thick oxide is grown in the unmasked areas, the nitride layer is removed, and the p-well is formed in the area without the thick oxide. Next, fox areas are formed by a conventional LOCOS process. U.S. Pat. No. 4,806,501 (Baldi et al.) teaches an isolation method where the n and p-wells are defined by two photoresist masks. However, these methods can be improved by reducing the number of process steps, reducing the surface topology and eliminating the use of photoresist as implant masks.